1. Field of the Invention
The invention relates to a process for producing wood particleboard by bonding together with the wood particles by means of a crosslinkable binder.
2. Background Art
To produce wood particleboard, the wood particles, for example wood fibers or wood chips, are glued together by means of an organic adhesive under pressure and at elevated temperature. The most important binders for chipboard production are urea-formaldehyde resins (UF resins). To produce moisture-resistant wood chipboard, phenol-formaldehyde resins (PF resins) are of great importance. Melamine-formaldehyde resins (MF resins) or mixtures of all three resin types mentioned are also used for improving the moisture resistance of wood particleboard. A disadvantage of these adhesives is that formaldehyde is emitted both during production of the particleboard and during use of the finished, pressed particleboard. Organic isocyanates have become established as formaldehyde-free binders for wood particleboard, in which case diphenylmethane 4,4xe2x80x2-diisocyanate (MDI) is generally used. However, handling MDI also requires special safety precautions during production of the wood particleboard because of the very reactive isocyanate groups and the associated irritation of skin and bronchial passages. A further disadvantage of these reactive resins becomes apparent in the production of MD and HD fiberboard: here, the isocyanate groups react in the blow-line during the direct addition of binder because of the elevated temperature, which leads to a loss of up to 25% of the binding potential.
Thermoplastic adhesives, for example those based on polyvinyl acetate, are likewise used for producing wood particleboard, as disclosed in U.S. Pat. No. 4,940,741. A problem with polyvinyl acetate binders is dimensional stability over the relatively high temperature range used in particleboard production, for example during removal from the mold. For this reason thermoplastic adhesives are frequently used in combination with the abovementioned formaldehyde condensate resins, with the abovementioned disadvantages. A low-VOC adhesive for bonding wood particleboard is described in DE-A 19941904. This adhesive is an aqueous mixture of high-viscosity polyvinyl alcohol, carboxyl-functional polyvinyl acetate and a water-soluble metal salt of a monobasic acid.
Formaldehyde-free, thermally curable, aqueous binders for producing wood particleboard are known, for example, from WO-A 97/31059. In this publication, a mixture of carboxyl-functional copolymer and an alkanolamine having at least two hydroxy groups is used. Aqueous adhesive compositions comprising polycarboxylic acid and hydroxyalkyl-substituted aminoaliphatics are described in WO-A 97/45461. WO-A 99/02591 discloses compositions comprising a carboxyl-functional copolymer and long-chain amines. A disadvantage of these systems, which crosslink via an esterification reaction, is that crosslinking occurs only in the water-free state, on drying.
It is an object of the invention to provide a process for producing wood particleboard in which the emission of pollutants such as formaldehyde is avoided but high-quality bonding is nevertheless obtained, and premature reaction of functional groups is largely prevented. These and other objects are achieved by the binder systems of the subject invention, which comprise a copolymer containing carboxyl, hydroxy, or NH-functional monomers, and an epoxy, N-methylol, isocyanate, or ethylene carbonate-functional crosslinker reactive with the functional groups of the copolymer.
The invention provides a process for producing wood particleboard, in which wood particles, if appropriate after a preceding comminution step, are treated with a crosslinkable binder and are subsequently pressed together under pressure and, if desired, elevated temperature to produce particleboard, wherein the binder comprises two constituents A) and B), where A) comprises a copolymer comprising one or more comonomer units selected from the group consisting of vinyl esters of unbranched or branched alkylcarboxylic acids having from 1 to 18 carbon atoms, acrylic esters or methacrylic esters of branched or unbranched alcohols having from 1 to 15 carbon atoms, dienes, olefins, vinylaromatics and vinyl halides, and from 0.1 to 50% by weight, based on the total weight of the copolymer, of one or more units comprising carboxyl, hydroxy or NH groups, and B) comprises at least one compound having at least two reactive groups which can react with the carboxyl, hydroxy or NH groups specified under A) and are selected from the group comprising epoxy, N-methylol, isocyanate and ethylene carbonate groups.
Suitable vinyl esters are vinyl esters of unbranched or branched carboxylic acids having from 1 to 18 carbon atoms. Preferred vinyl esters are vinyl acetate, vinyl propionate, vinyl butyrate, vinyl 2-ethylhexanoate, vinyl laurate, 1-methylvinyl acetate, vinyl pivalate and vinyl esters of xcex1-branched monocarboxylic acids having from 5 to 11 carbon atoms, for example VeoVa9R or VeoVa10R vinyl esters available from Shell. Particular preference is given to vinyl acetate.
Suitable monomers from the group consisting of esters of acrylic acid or methacrylic acid are esters of unbranched or branched alcohols having from 1 to 15 carbon atoms. Preferred methacrylic esters or acrylic esters are methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, n-butyl acrylate, n-butyl methacrylate, t-butyl acrylate, t-butyl methacrylate, 2-ethylhexyl acrylate and norbornyl acrylate. Particular preference is given to methyl acrylate, methyl methacrylate, n-butyl acrylate, 2-ethylhexyl acrylate and norbornyl acrylate.
Suitable dienes are 1,3-butadiene and isoprene. Examples of copolymerizable olefins are ethene and propene. Vinylaromatics which can be copolymerized include styrene and vinyltoluene. From the group of vinyl halides, use is usually made of vinyl chloride.
Suitable carboxyl-functional comonomers for copolymer A) are ethylenically unsaturated monocarboxylic and dicarboxylic acids, preferably acrylic acid, methacrylic acid, crotonic acid, itaconic acid, fumaric acid and maleic acid. The carboxyl function can also be introduced into the copolymer A) by copolymerization of maleic anhydride. Suitable hydroxy-functional comonomers are hydroxyalkyl acrylates and hydroxyalkyl methacrylates having a C1-C8-alkyl radical, preferably hydroxyethyl acrylate and methacrylate, hydroxypropyl acrylate and methacrylate, and hydroxybutyl acrylate and methacrylate. Suitable NH-functional comonomers are (meth)acrylamide, diacetoneacrylamide, maleimide, amides of monoalkyl maleates and fumarates, diamides of maleic and fumaric acids, amides of monovinyl glutarates and succinates, and amides of monoallyl glutarates and succinates. The NH-functional units can also be introduced into the copolymer A) as aminofunctional oligomers containing primary or secondary NH groups, preferably those containing primary NH groups such as Jeffamine(copyright) amines. The proportion of functional units in copolymer A) is preferably from 1 to 30% by weight, more preferably from 5 to 20% by weight, in each case based on the total weight of the copolymer.
Preference is given to the following base polymer compositions for the copolymer A) which in addition to the base monomers, further comprise the abovementioned functional groups: vinyl acetate polymers; vinyl ester-ethylene copolymers such as vinyl acetate-ethylene copolymers; vinyl ester-ethylene-vinyl chloride copolymers in which the vinyl esters present are preferably vinyl acetate and/or vinyl propionate and/or one or more copolymerizable vinyl esters such as vinyl laurate, vinyl pivalate, vinyl 2-ethylhexanoate, vinyl esters of alpha-branched carboxylic acids having from 5 to 11 carbon atoms, in particular vinyl esters of Versatic acid, i.e. VeoVa9R and/or VeoVa10R; vinyl acetate copolymers with one or more copolymerizable vinyl esters such as vinyl laurate, vinyl pivalate, vinyl 2-ethylhexanoate, vinyl esters of alpha-branched carboxylic acids having from 5 to 11 carbon atoms, in particular vinyl esters of Versatic acid (VeoVa9R, VeoVa10R), which may further comprise ethylene; vinyl ester-acrylic ester copolymers, in particular with vinyl acetate and butyl acrylate and/or 2-ethylhexyl acrylate, which may further comprise ethylene; vinyl ester-acrylic ester copolymers with vinyl acetate and/or vinyl laurate and/or vinyl esters of Versatic acid and acrylic esters, in particular butyl acrylate or 2-ethylhexyl acrylate, which may further comprise ethylene.
Particular preference is given to (meth)acrylic ester polymers and styrene polymers: copolymers with n-butyl acrylate and/or 2-ethylhexyl acrylate; copolymers of methyl methacrylate with butyl acrylate and/or 2-ethylhexyl acrylate, and/or 1,3-butadiene; styrene-1,3-butadiene copolymers and styrene-(meth)acrylic ester copolymers such as styrene-butyl acrylate, styrene-methyl methacrylate-butyl acrylate or styrene-2-ethylhexyl acrylate, where n-, iso- and t-butyl acrylate can be used as butyl acrylate.
Most preferred are compositions containing a carboxyl-functional styrene-n-butyl acrylate copolymer and/or a carboxyl-functional styrene-methyl methacrylate-n-butyl acrylate copolymer as copolymer A).
Suitable crosslinkers B) include non-thermoplastic compounds such as epoxide crosslinkers of the bisphenol A type, i.e. condensation products of bisphenol A and epichlorohydrin or methylepichlorohydrin. Such epoxide cross-linkers are commercially available, for example under the trade names Epicote and Eurepox. Also suitable are blocked or unblocked diisocyanates which may also be commercial products, for example m-tetramethylxylene diisocyanate (TMXDI) and diphenylmethane diisocyanate (MDI).
Further suitable crosslinkers B) are copolymers which bear epoxy, N-methylol, ethylene carbonate or isocyanate group or combinations of these groups. The polymer compositions for the crosslinker B) may include the same comonomers described as suitable for copolymer A). Preference is given to the polymer compositions mentioned as preferred for the copolymer A) which contain comonomer units comprising epoxy, N-methylol, ethylene carbonate and/or isocyanate groups instead of the carboxyl, hydroxy or NH groups. Particular preference is given to (meth)acrylic ester polymers and styrene polymers, for example copolymers with N-butyl acrylate and/or 2-ethylhexyl acrylate; copolymers of methyl methacrylate with butyl acrylate and/or 2-ethylhexyl acrylate and/or 1,3-butadiene; styrene-1,3-butadiene copolymers and styrene-(meth)acrylic ester copolymers such as styrene-butyl acrylate, styrene-methyl methacrylate-butyl acrylate or styrene-2-ethylhexyl acrylate, where n-, iso-, t-butyl acrylate can be used as butyl acrylate.
The content of epoxy-, N-methylol-, ethylene carbonate- and isocyanate-functional comonomers is from 0.1 to 50% by weight, preferably from 1 to 30% by weight, particularly preferably from 5 to 20% by weight, in each case based on the total weight of the copolymer B). Suitable epoxide-functional comonomers are glycidyl acrylate, glycidyl methacrylate, allyl glycidyl ether, vinyl glycidyl ether, vinylcyclohexene oxide, limonene oxide, myrcene oxide, caryophyllene oxide, styrenes and vinyltoluenes substituted in the aromatic ring by a glycidyl group, and vinyl benzoates substituted in the aromatic ring by a glycidyl group. Suitable isocyanate-functional comonomers are 2-methyl-2-isocyanatopropyl methacrylate and isopropenyldimethylbenzyl isocyanate (TMI). Suitable N-methylol-functional comonomers are N-methylolacrylamide (NMA), N-methylolmethacrylamide, allyl N-methylcarbamate, alkyl ethers such as the isobutoxy ether or esters of N-methylolacrylamide, of N-methylolmethacrylamide and of allyl N-methylcarbamate.
The most preferred crosslinkers B) are the epoxide crosslinkers of the bisphenol A type and epoxide-functional copolymers such as styrene-(meth)acrylic ester copolymers containing glycidyl methacrylate units.
The preparation of copolymer A) and, if desired, the polymeric crosslinker B) can be carried out by means of known free-radical-initiated polymerization processes, for example by means of solution polymerization or aqueous emulsion polymerization. For drying, it is possible to employ all customary drying methods: spray drying, drum drying, freeze drying, belt drying, or coagulation with subsequent fluidized-bed drying. Preference is given to using spray drying and drum drying methods. In the abovementioned copolymers, the comonomers are preferably copolymerized in such a ratio that both copolymer A) and copolymer B) have a melting point or a glass transition temperature xe2x80x9cTgxe2x80x9d of  greater than 30xc2x0 C., preferably xe2x89xa745xc2x0 C. The glass transition temperature Tg and the melting point of the polymers can be determined conventionally by means of differential scanning calorimetry (DSC). The Tg can also be calculated approximately by means of the Fox equation. According to T. G. Fox, BULL. AM. PHYSICS Soc. 1, 3, page 123 (1956): 1/Tg=x1/Tg1+x2/Tg2 +. . . +xn/Tgn, where xn is the mass fraction (% by weight/100) of the monomer n and Tgn is the glass transition temperature in degrees Kelvin of the homopolymer of the monomer n. Tg values for homopolymers are given in the POLYMER HANDBOOK, 2nd Edition, J. Wiley and Sons, New York (1975).
The two constituents A) and B) are present in such a ratio that the molar ratio of functional comonomer units of constituent A) to those of constituent B) is in the range from 5:1 to 1:5. Particular preference is given to equimolar ratios of the functional comonomer units. If appropriately functionalized copolymers have been used for each of the constituents A) and B), they are selected so that they are compatible with one another, i.e. are miscible with one another on a molecular level. For this reason, the copolymers A) and B) present in the combination are usually chosen so that they are made up predominantly of the same comonomer units, apart from the functional comonomer units. The greatest preference is therefore given to compositions comprising carboxyl-functional styrene-n-butyl acrylate and/or styrene-methyl methacrylate-n-butyl acrylate copolymers as constituent A) and styrene-n-butyl acrylate and/or styrene-methyl methacrylate-n-butyl acrylate copolymer(s) containing glycidyl methacrylate units as constituent B).
The constituents A) and B) can be employed in dry, pulverulent form (dry gluing), or in the form of an aqueous dispersion or an aqueous solution (wet gluing). The constituents A) and B) can both be used as powder or both be used as aqueous solution or aqueous dispersion. It is also possible to use powders, aqueous solutions or aqueous dispersions in which both constituents A) and B) are present in admixture. Finally, it is also possible to use any combination of powders, solutions and dispersions in each of which one constituent is present. Depending on the process variant, the binder constituents A) and B) can be used separately or in admixture. When using pulverulent binders, the fibers may be wetted with water or an olefin wax emulsion. For this purpose, from 2 to 10% by weight of water and/or olefin wax emulsion, based on binder, may be sprayed onto the fibers or chips.
The production of wood particleboard is described in detail in Ernst Deppe, TASCHENBUCH DER SPANPLATTENTECHNIK, 3rd edition, 1991. To produce the wood particleboard, the wood particles are used as chips or fibers. The fibers or chips, which may optionally have been predried, are mixed with binder in customary mixing vessels such as ploughshare mixers. In general, the binder composition is used in an amount of from 5 to 30% by weight, preferably in an amount of from 7 to 15% by weight, in each case based on wood particle weight (solid/solid). The fibers or chips which have been mixed with binder are sprinkled uniformly on a molding belt and, if desired, subjected to preliminary cold pressing. The fiber layer is subsequently pressed by means of hot platens at temperatures of from 150xc2x0 C. to 250xc2x0 C. and under a pressure of from 10 to 100 bar to form boards.
To produce oriented strand board (OSB), strands having a length/width ratio of generally from 5:1 to 10:1 are used and are treated wet or dry with binder as described above and are then oriented and laid down in layers, after which they are hot pressed under the conditions mentioned.
In the production of medium density fiberboard (MDF) and high density fiberboard (HDF), the fibers are pretreated hydrothermally in a refiner unit, i.e. boiled and milled at elevated temperature under steam pressure, subsequently fluidized in a stream of steam in a blow-line, then dried in a drying tube, optionally precompacted in a cold press and finally pressed under pressure, preferably from 10 to 100 bar, and, if desired, elevated temperature, preferably from 150xc2x0 C. to 250xc2x0 C., to produce boards. To apply the binder, the fibers or chips, after milling while still under steam pressure and at temperatures of from 120xc2x0 C. to 150xc2x0 C., are sprayed with an aqueous dispersion or aqueous solution of the binder composition in the blow-line or in the drying tube or, when using pulverulent binder compositions, are mixed with the pulverent binder. The fibers which have been treated with binder are subsequently dried. As an alternative, the wood fibers can also be dried without having been treated with binder and are then mixed in the dry state with the binder in a suitable mixer.
Particular preference is given to a process in which the binder constituents A) and B) are used separately as two components (2-component system) for producing MDF and HDF. For this purpose, one component of the binder can be introduced into the mill of the refiner unit during the milling step. The greatest preference is given to a procedure in which a constituent of the binder, in particular constituent A), is introduced into the refiner unit before the mill, in the mill, or shortly after the mill in the blow-line, preferably in the first third thereof. The second constituent, namely the crosslinker component B), is introduced in a later stage of the process. This can be carried out at the end of the blow-line of the refiner unit, preferably in the last third thereof, during drying of the fibers in the drying tube or after drying of the fibers. The advantage of this process variant is that the crosslinker component B) is added in a process step in which thermal stress is lower, and thus much less premature crosslinking occurs.